1. Field of the Invention
The present invention relates to multi-carrier transmission systems using orthogonal carriers in which a receiver sampling clock is synchronised to a transmitter sampling clock by phase locking to a pilot carrier. In particular, the invention relates to receivers and transceivers for use with such systems and a method of synchronising a receiver sampling clock with a transmitter sampling clock.
2. Discussion of the Background
The demand for provision of multi-media and other bandwidth services over telecommunications networks has created a need to transmit high bit rate traffic over copper pairs. This requirement has led to the development of a number of different transmission schemes, such as, ADSL and VDSL. One of the more likely modulation systems for all these transmission schemes is a line code known as DMT (discrete multi-tone), which bears some resemblance to orthogonal frequency division multiplex, and is a spread spectrum transmission technique.
In discrete multi-tone transmission, the available bandwidth is divided into a plurality of sub-channels each with a small bandwidth, 4 kHz perhaps. Traffic is allocated to the different sub-channels in dependence on noise power and transmission loss in each sub-channel. Each channel carries multi-level pulses capable of representing up to 11 data bits. Poor quality channels carry fewer bits, or may be completely shut down.
Because inter pair interference in copper pair cables is higher where data is transmitted in both directions, i.e. symmetric duplex, a number of transmission schemes have proposed the use of asymmetric schemes in which high data rates are transmitted in one direction only. Such schemes meet many of the demands for high bandwidth services, such as, video-on-demand but, in the long term, symmetric duplex systems will be required.
VDSL technology resembles ADSL to a large degree, although ADSL must cater for much larger dynamic ranges and is considerably more complex as a result. VDSL is lower in cost and lower in power, and premises VDSL units need to implement a physical layer media access control for multiplexing upstream data.
Four line codes have been proposed for VDSL:
CAP; Carrierless AM/PM, a version of suppressed carrier QAM, for passive NT configurations, CAP would use QPSK upstream and a type of TDMA for multiplexing (although CAP does not preclude an FDM approach to upstream multiplexing);
DMT; Discrete Multi-Tone, a multi-carrier system using Discrete Fourier Transforms to create and demodulate individual carriers, for passive NT configurations; DMT would use FDM for upstream multiplexing (although DMT does not preclude a TDMA multiplexing strategy);
DWMT; Discrete Wavelet Multi-Tone, a multi-carrier system using Wavelet Transforms to create and demodulate individual carriers, DWMT also uses FDM for upstream multiplexing, but also allows TDMA; and
SLC; Simple Line Code, a version of four-level baseband signalling that filters the base band and restores it at the receiver, for passive NT configurations; SLC would most likely use TDMA for upstream multiplexing, although FDM is possible.
Early versions of VDSL will use frequency division multiplexing to separate downstream from upstream channels and both of them from POTS and ISDN. Echo cancellation may be required for later generation systems featuring symmetric data rates. A rather substantial distance, in frequency, will be maintained between the lowest data channel and POTS to enable very simple and cost effective POTS splitters. Normal practice would locate the downstream channel above the upstream channel. However, the DAVIC specification reverses this order to enable premises distribution of VDSL signals over coaxial cable systems.
In a multi-carrier system, such as a DMT system, a receiver must be able to recover a sampling clock that is very accurately synchronized to a transmitter sampling clock. A known method, for achieving synchronization, uses a reserved carrier, the pilot carrier, which is transmitted with a fixed phase. The receiver sampling clock is then phase locked to the pilot carrier. Frame timing must also be recovered. This can be achieved by using a correlation technique.
It is an object of the present invention, in a multi-carrier transmission system, to provide an improved method of recovering a sample clock and phase locking the sampling clock to a pilot carrier, thereby synchronising a receiver sample clock to a transmitter sample clock.
It is a further object of the present invention to provide a multi-carrier transmission system in which a pilot carrier is transmitted, incorporating an improved technique for phase locking a sampling clock, in a receiver, to said pilot carrier, thereby synchronising said receiver sample clock to a transmitter sample clock.
It is a yet further object of the present invention to provide a receiver, which may form one element of a transceiver, for use with a multi-carrier transmission system, said receiver and/or transceiver employing an improved technique for phase locking a sampling clock, in said receiver, to a pilot carrier, thereby synchronising said receiver sample clock to a transmitter sample clock.
According to a first aspect of the present invention, there is provided a receiver, for use with a multi-carrier transmission system using orthogonal carriers, in which a receiver sampling clock is synchronized with a transmitter sampling clock by phase locking said receiver sampling clock to a pilot carrier, characterised in that said receiver includes:
a first feedback loop adapted to align said receiver""s frame timing by using a frame timing deviation signal to control an oscillator means adapted to produce said receiver sampling clock; and
a second feedback loop adapted to compensate for channel effects by using a signal representative of an argument estimate of said pilot carrier to control said oscillator means.
Said oscillator means may be a VCXO.
Said multi-carrier transmission system may be a DMT transmission system.
Said multi-carrier transmission system may be a DMT based VDSL system.
Said receiver may include correlator means connected to peak position estimator means adapted to produce said frame timing deviation signal.
Said receiver may include pilot argument estimation means adapted to produce said signal representative of said pilot argument estimate.
Said peak position estimation means and said pilot argument estimation means may each be connected to feedback controller means which may be connected, via a D/A, to said oscillator means.
Said feedback controller means may comprise two feedback controllers whose outputs are summed to produce a control signal for said oscillator means.
Said signal representative of said pilot argument estimate may function as an equaliser parameter and cause said oscillator means to be adjusted in a manner which compensates for channel effects.
Said receiver may operate in accordance with a synchronization process having the following elements:
frame alignment by adjustment of said oscillator means in response to a frame timing deviation signal;
equalisation by adjustment of said oscillator means in response to said signal representative of said pilot carrier argument; and
after a settling down period has elapsed, adjustment of said oscillator means in response to said signal representative of said pilot carrier argument estimate, only.
Said signal representative of said pilot carrier argument estimate may be employed to equalize time variations in channel characteristics for a channel over which said pilot carrier is transmitted.
Monitoring means may be provided to monitor frame timing deviations and to initiate resynchronization if said frame timing deviations exceed a large predetermined value.
According to a second aspect of the present invention, there is provided a transceiver, including a transmitter and a receiver, characterised in that said receiver is a receiver as set forth in the preceding paragraphs.
According to a third aspect of the present invention, there is provided a multi-carrier transmission system using orthogonal carriers, in which a receiver sampling clock is phase locked to a pilot carrier, characterised in that said multi-carrier transmission system includes two transceivers as set forth in the preceding paragraph.
According to a fourth aspect of the present invention, there is provided, in a multi-carrier system in which a transmitter and a receiver are synchronised by means of a pilot carrier transmitted by said transmitter to said receiver, a method of recovering, at said receiver, said transmitter""s sampling clock from said pilot signal and synchronising said transmitter with said receiver, characterised by the steps of:
deriving a frame timing deviation signal;
using said frame timing deviation signal to align a received frame structure with a transmitted frame structure; and
deriving an estimate of said pilot carrier""s argument and using it as an equalisation parameter to compensate for channel influence.
According to a fifth aspect of the present invention, there is provided, in a multi-carrier transmission system using orthogonal carriers, in which a receiver sampling clock is synchronized with a transmitter sampling clock by phase locking said receiver sampling clock to a pilot carrier transmitted by said transmitter, a method of synchronising said receiver""s sampling clock to said transmitter sampling clock, characterised by the steps of:
aligning said receiver""s frame timing by using a frame timing deviation signal to control said receiver""s sampling clock; and
compensating for channel effects by using a signal representative of an argument estimate of said pilot carrier to control said receiver""s sampling clock.
Said frame timing deviation signal may be derived, by correlation, from a stream of received time domain data.
Said frame timing deviation signal may be added to said signal representative of an argument estimate of said pilot carrier to produce a receiver sampling clock control signal.
Said signal representative of said pilot argument estimate may function as an equaliser parameter and may cause said oscillator means to be adjusted in a manner which compensates for channel effects.
Said method may include the following elements:
frame alignment, between said receiver and said transmitter, by adjustment of said receiver""s sampling clock in response to a frame timing deviation signal;
equalisation of a channel over which said pilot signal is transmitted by adjustment of said receiver""s sampling clock in response to said signal representative of said pilot carrier argument; and
after a settling down period has elapsed, adjustment of said receiver""s sampling clock in response to said signal representative of said pilot carrier argument, only.
Said signal representative of said pilot carrier argument estimate may be employed to equalize for time variations in channel characteristics in a channel over which said pilot carrier is transmitted.
Said frame timing deviations may be monitored and a resynchronization initiated if said frame timing deviations exceed a large predetermined value.